Why is wombat poop cube-shaped?

Wombats are the only animals in the world that produce cube-shaped scat. But how and why do they do it? Scientists now have a better idea.

By Tik Root

A young common wombat (
Vombatus ursinus tasmaniensus) at the Healesville Sanctuary. These animals produce some of the animal kingdom's strangest-shaped scat.

Photograph by Joel Sartore, National Geographic Photo Ark

Wombats are a burrowing animal native to Australia perhaps best-known
for being, well, pudgy—and quite cute.
But there’s something you might not know about these adorable marsupials:
Wombats are the only animals in the world that produce cube-shaped poop.

While this peculiarity has sparked much interest and debate, actual research
into the intricacies of wombat scat has been scant. That’s left scientists
largely in the dark about the phenomenon – until recently.

Earlier this year, Patricia Yang,
a researcher at the Georgia Institute of Technology who specializes in
bodily fluids, started to look into the topic more closely after hearing
about it at a conference.

“I didn't really believe it,” Yang says. But after confirming that it
is, indeed, a fact, she began trying to figure out why, and how, wombats
poop in cubes.

“People have had all sorts of theories,” says Mike Swinbourne,
a wombat expert at the University of Adelaide in Australia. One popular
postulate is that wombats make cubes so that they can stack them to mark
their territory, without the pieces rolling away. But Swinbourne says that’s
a misconception.

Video: Cubic wombat scat

While wombats do use their scat to mark territory, “it's not like they're
trying to build little brick pyramids,” he says. “They just poop where
they poop.”

Instead, Swinbourne says the cubic shape is more likely related to the
dry environments that most wombats live in. “They have to really squeeze
every drop of moisture out [of their food],” he said. And sometimes, in
zoos, where the animals have readier access to hydration, Swinbourne says
their scat is less cubic. Being dry helps the scats form more rigid shapes
with sharper angles.

Moisture plays a role, but “it's also a factor of the primary digestive
tract,” adds Bill Zeigler, senior vice president for animal programs at
Brookfield Zoo in Chicago, which has had wombats since 1969. Peter Clements,
the president of the organization Wombats SA in Southern Australia, concurs,
speculating that it’s a combination of the two.

Finding a more concrete answer, however, hasn’t been easy. It took Yang
and her colleagues months just to get ahold of wombat innards for their
study. No zoos in North America had any, so Yang had the intestines of
two roadkill wombats shipped from Australia. She wasn’t sure what to expect
when they arrived.

“At first I thought they maybe have square anus, or maybe [the cube] forms
right around the stomach,” she said. But neither of those hypotheses turned
out to be the case. What she found to be more important was how the wombat
intestines’ stretched.

As food is digested it moves through the gut, and pressure from the intestine
helps sculpt the feces – meaning that the shape of the intestine will affect
the shape of a dropping. So Yang and the team expanded both wombat and
pig intestines with a balloon to measure and compare their elasticities
(or stretchiness).

The pig intestine had a relatively uniform elasticity, which would explain
the animal’s rounder poo. The wombat intestines, however, had a much more
irregular shape. Yang observed two distinct ravine-like grooves, where
the intestine is stretchier, which she believes helps shape wombat feces
into cubic scat.

“It's really the first time I've ever seen anybody come up with a good
biological, physiological explanation,” said Swinbourne, who reviewed the
draft. Clements, who also read the early study, added, “I think this is
a useful contribution but more explanation of a possible mechanism would
be helpful.”

Yang agrees that there are still a host of questions to answer and says
her research is ongoing. Her next task is to figure out why only two grooves
produce a cube, as opposed to needing four. But even the initial findings
imply broader implications for sectors such as manufacturing.

Cubes, Yang says, are very rare in nature. “We currently have only two
methods to manufacture cubes,” she said, explaining that humans either
mold cubes from soft materials, or cut them from harder objects.

“Wombats have a third way.”

PUBLISHED November 19, 2018

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